My Darling Soulmate
by Luna Darkside
Summary: Shinichi isn't expecting his soulmate to be a criminal. Similarly, Kaito isn't expecting his soulmate to be six years old. Their lives are hard. /ShinKai & KaiShin, firstwordstattoo!soulmates!au, oneshot, complete/


_Just an idea I had while reading a "first words written on wrist" soulmate AU, which is an AU in which a character has the first words they exchange with their soulmate written somewhere on their body, presumably in their soulmate's handwriting. Considering that Shinichi and Kaito met as Conan and Kid, I thought this would be fun._

 _Although this is operating under the notion that even though the clock tower heist is technically Shinichi and Kaito's first meeting, they didn't actually talk to each other (I could be wrong, though – I haven't watched that episode in a while), so I used their meeting from episode 76. Yeah. Everything after their first meeting, though, departs from canon._

 _Also I've aged Kaito, Shinichi, and Ran up to eighteen because sixteen feels too young for soulmates, in my opinion. Conan, however, is still six._

 _Wow that's a lot of background info. I'm also going to warn that this is a looooot of pointless fluff, because I didn't know where I was going to go after the first meeting and everything sort of spiraled out of control. Yeah. Enjoy? - Luna_

 **love is composed of a single soul inhabiting two bodies.**  
– aristotle

According to legend (or rather, according to Shinichi's mother, who may not be the most accurate source), there was once a pair of soulmates who were cursed to never verbally speak to each other (because they angered the gods or something; Yukiko wasn't too clear on that point). Because of their affliction, they had to communicate by writing on each other's arms, legs, necks, torsos, everywhere they could.

By the time they died (together and of old age), both were covered in the words they had exchanged, a lasting reminder of their bond. And to commemorate their love, all their descendants were blessed (plagued, in Shinichi's opinion) with the first words they would share with their soulmate written somewhere on their body.

Shinichi never understood the appeal of the legend, to be honest. It also never made sense to him. It wasn't as if words written on skin were hereditary or anything, so why would their children have them? Also, why hadn't they just used paper instead of writing all over each other in the first place?

Every time he voiced this opinion, however, his mother would frown and scold him about destroying the _romance_ of finding your soulmate and how he would understand when he was older.

Well, Shinichi was older now, and he still didn't understand what was so great about the soulmates thing. Sure, meeting the other half of your soul and all that probably would be nice, but still. Soulmates were totally overrated.

His irritation about soulmates had _absolutely nothing_ to do with the fact that his own soulmate mark, comprised of messily scrawled characters in indigo across the knuckles of his right hand, read _Yo, kid. What are you doing at a place like this?_ , which Shinichi had deduced meant his soulmate was either a) a condescending asshole or b) a lot older than him.

 _Absolutely nothing_ to do with that. At all.

Okay. Fine. So Shinichi was bitter. When he had actually been young enough to qualify as a kid, he had been more excited about his soulmate, but as he'd gotten older and nobody had greeted him with "Yo, kid," he'd started losing hope. Now that he was a respectable eighteen years old, what kind of person would greet him with "Yo, kid," anyway?

And then Shinichi had gotten shrunken and things started making a little more sense – the "kid" part, at least. He was still caught off guard the night he met the other half of his soul.

Because seriously. He had _not_ been expecting his soulmate to be _Kaitou Kid_.

* * *

For the longest time, Kaito had mixed feelings regarding his soulmate mark. It wasn't particularly hard to hide – located on the inside of the thumb on his left hand – and it was pretty enough, composed of blocky, utilitarian lettering in a bright azure color.

What it actually _said_ , though, was a different story.

 _Fireworks!_

Kaito didn't know how to feel about that. Did that mean his soulmate was a fireworks enthusiast? Would they meet at a fireworks show? There were too many possibilities to consider with just _Fireworks!_ to go off.

"You're overthinking it. You'll meet them when the time is right," Aoko liked to tell him, and Kaito would roll his eyes. It was easy enough for her, having already found her soulmate in _Hakuba_ , of all people. Kaito wondered what it said about her that the other half of her soul was a complete and utter bastard, but he knew better than to mention it. Aoko didn't like it when he criticized her soulmate, oddly enough.

But anyway. Kaito didn't quite know what the correct response to his soulmate mark was, and so he kept it covered most of the time, hidden under Band-Aids and stacks of gaudy thumb rings.

And one night he put on his cape and monocle and poker face, went out with the intent of completing a standard heist, and was instead confronted by his soulmate, who was, incidentally, _six years old_.

Kaito's life was _suffering_.

* * *

So Shinichi's soulmate was a criminal. What about it. He wasn't disappointed or irritated or anything. He was _definitely_ not, like, sulking or anything.

Shinichi wasn't sure whom he was trying to convince.

He sank farther into the professor's couch, glared at the copy of _A Study in Scarlet_ that lay open across his lap in an attempt at distraction, and mourned his previous ignorance. It really was true, that "ignorance is bliss" thing.

"Professor," Shinichi groaned loudly when the cycle of _yay soulmate! / no he's a criminal and I don't want him / but he's your soulmate so you love him / he's a_ criminal, _goddammit / but soulmate!_ whirling on spin-dry in his head got to be too much. He buried his face in his hands, glasses squishing against the bridge of his nose rather painfully (he was past caring). "This is a serious problem. This is a _moral dilemma_."

In the kitchen, Agasa continued to putter about, carefree as always as he either made coffee or built a hydrogen bomb. It was hard to tell with him, sometimes. "I think it's a good thing, Shinichi," he chirped, far more cheerfully than the whole messed up situation really deserved. "One character trait doesn't define him completely. He could still be a good person."

"But he's a criminal," Shinichi whimpered, kicking his annoyingly short legs as fiercely as he could. Abruptly feeling like an actual six-year-old throwing a tantrum, he stopped. "I don't know about you, but I feel like that's a pretty big part of his personality. You know." He waved a hand weakly, wearily. "A disregard for the law and all."

"Sure," Agasa conceded, trotting over with a mug of steaming coffee (so he _had_ been making coffee), which he set on the coffee table. "But you yourself said that he _did_ impress you. And he _did_ escape from you."

Scowling, Shinichi reached for the mug, noting (with some displeasure) how the cup dwarfed his tiny hands. _Ugh_. "I mean," he grumbled, feeling out of sorts and uncomfortable with Agasa's knowing smile looming large in his periphery, "he _is_ impressive. His skill set is, at least." He thought of how effortlessly Kid had mimicked Nakamori's voice, how ridiculously stunning he'd looked backlit with searchlights, how smoothly his voice had cut through the smoke and flashing lights even as he'd mocked Shinichi.

How Shinichi had eyed the crooked curve of Kid's mouth until he'd gotten it memorized perfectly, could probably graph it if he wanted. How he'd wondered absently if their hearts beat in sync like the rumors about soulmates said. How he'd gotten the insane urge to just reach out and map the exact contours of Kid's face with his fingers. Maybe it was just the knowledge that Kid literally completed him, but there was still _something_ about him that Shinichi would've found enthralling regardless of soulmate bonds.

Upon realizing Agasa was watching him with something like satisfaction, Shinichi broke into a hacking fit. "I mean only – only his skill set is impressive," he mumbled, and, feeling his face heat up as Agasa hummed meaningfully, rolled his eyes and took a sip of the coffee. Predictably, he scalded his tongue and spent the next five minutes wallowing in his misery.

* * *

Rubbing at his thumb, Kaito slid into his seat beside Aoko with considerably less grace than usual and stared fixedly at her until she noticed. When she turned away from Hakuba to lift her eyebrows at him in a _what do you want_ sort of way, he blurted out, "You've got a soulmate." He meant to follow up with "And so you're an expert on soulmates, right?" but the words got stuck in his throat at the fleeting memory of precocious Edogawa Conan glaring at him from behind oversized glasses. Oh God, he was so _screwed_. He was _sickening_. There was definitely something wrong with him.

In the ensuing silence, Aoko narrowed her eyes at him. "I do," she agreed, guarded, and Kaito dropped his head against the desk. He had a migraine.

"I have a…" He coughed, looking away. He could feel Hakuba joining Aoko's "stare at Kaito until he feels uncomfortable" club. "A… theoretical question."

"Right," Aoko prompted when Kaito didn't immediately continue. "So."

"What if," Kaito finally managed, once he had summoned enough courage to look the two of them in the faces, "what if there was an… age gap between soulmates."

Frowning, Aoko opened her mouth, but Hakuba cut in with, "That's not uncommon, Kuroba-kun." His tone fairly screamed _why are you bothering us with this_.

"No, I mean." Kaito cleared his throat, suddenly finding the wall behind Hakuba's head very, very interesting. "What if there's, like."

"Like," Aoko echoed with a hint of impatience after Kaito paused for too long.

Kaito lifted his gaze to stare mournfully at them. "What if there's, like, an age gap of, say, uh… twelve years or so."

Behind Aoko, Hakuba made a choked noise as he seemed to figure out where the conversation was heading. Kaito winced.

Aoko, oblivious, just slanted her head at him in bewilderment. "I mean, that's not _super_ common, but it's not _that_ weird either." She was beginning to look at him with her patented _do I need to call you an ambulance or suggest a psychiatrist_ expression, which was quite familiar after ten years of friendship.

"But." Kaito swallowed dryly. Hakuba was staring at him with burgeoning concern-mixed-horror. "What if the person is, um, eighteen, and the soulmate is… six?"

There was an awkward silence for about three seconds before Aoko laughed loudly. Her eyes were both incredulous and – well, mostly incredulous. "Oh, you – that's – oh. Wow. Um." She clamped her mouth shut.

"This is all – hypothetical, of course," Kaito tried, and both Hakuba and Aoko began nodding in a way that meant they were definitely not convinced.

"Right, hypothetical. Well, my advice would be to – to wait for them to be... uh... legal, I… think," Aoko eventually managed, looking as wild-eyed and out of her depth as Kaito inwardly felt.

"I… okay," Kaito muttered and waved them off. He could feel them discussing him in whispers, but he couldn't bring himself to care.

He also couldn't stop thinking about the moonlit luminosity of Edogawa Conan's eyes, too old and too knowing even behind glass, or the overblown, childish way his voice rose and fell as he said "Fireworks!" and set off the fireworks show, how he'd looked so oddly fierce as he glared when Kaito made his escape.

Kaito groaned and dropped his head against his desk. He felt uncontrollably disgusting thinking about a _kid_ like that. There was most definitely something wrong with him.

* * *

The next time Shinichi confronted Kid, during the second part of Kid's Black Star heist, Kid was dressed as Ran in a truly ironic twist of fate, considering Kid was his soulmate and Ran was his (ex)-crush. Shinichi swallowed against the conflicting feelings that began to war in his stomach, trying his hardest not to flinch as he explained his reasoning. Even though he threatened to put Kid in prison, he wondered if he'd be able to do just that when Kid was evoking some seriously Freudian feelings with the disguise.

Thankfully, though, Kid eliminated the distraction by pulling off the latex mask and tucking it under his arm. Even in the dim light of the engine room, still clad in Ran's belted red sheath dress, he was resplendent, the personification of a sunbeam.

Or maybe that was just the soulmate bond trying to convince him to marry Kid or whatever soulmate bonds tried to do and _he should ignore it_ , Shinichi reminded himself harshly, and refocused his attention on the matter at hand.

Kid was regarding him carefully. "I'll give you back the pearl," he shrugged, bare shoulders rippling as he reached up to tug the pearl free of the pin with a handkerchief and toss it in Shinichi's direction. Shinichi determinedly did not stare as he lifted a foot to brace it against the wall and his dress rode up with the motion, exposing his thigh. "But I think we need to discuss some things."

"Like… the weather?" Shinichi hedged, abruptly feeling as if the situation had slipped out of his control.

"No," Kid said, rolling his eyes, and leaned back against the wall. "You know. The…" He cleared his throat. "Soulmate… thing."

"I don't know what you're talking about," Shinichi attempted in a pitifully childish tone, but Kid leveled him with a flat stare.

"Don't try that 'innocent kid' thing with me – you literally just told me all your deductions about how you figured out who I was. You're intelligent enough to know what I'm talking about. We're… soulmates," he stated, tone stiff and awkward in distinct contrast to his flamboyant persona, and Shinichi groaned.

"I was rather hoping it was some kind of – of mistake," he mumbled, scuffing the toe of his sneaker against the floor. The boat lurched minutely, matching Shinichi's stomach.

"You're one to talk," Kid replied. When Shinichi peered over at him, he looked positively miserable. "I'm not – you're just a _kid_ , and I'm…" He exhaled slowly, avoiding Shinichi's eyes. "If we had met when you were older, maybe it would've been less… you know."

Goggle-eyed, Shinichi gaped at him until he realized what things must look like to Kid. He _was_ in the body of a six-year-old; of _course_ that would've been a source of horror for Kid, who Shinichi estimated was probably around Shinichi's actual age. Twelve year age gaps, while certainly possible, were somewhat uncommon and definitely awkward when you were eighteen and your soulmate was six.

He bit his bottom lip, considering. Kid wasn't really _connected_ to him (other than by the whole soul thing) – nobody _knew_ they were soulmates, at least not yet, and he doubted the Black Organization would go after him. Probably. Maybe.

And, well, the way Kid seemed so – so _self-loathing_ , almost, as if he couldn't believe that he felt so strongly about a kid… that sort of crushed Shinichi's heart, so. It was a good a reason as any.

"About that," Shinichi started, shifting a little. "I'm not actually six."

For a moment, Kid just blinked at him, and then he sighed. His hair, messy and spiky and flat in some places, fell over his eyes as he shook his head. "Let me guess. You're actually seven. Sorry, I misjudged your age."

"No," Shinichi stammered, feeling heart palpitations starting in his chest, "no, I mean I'm actually eighteen."

Kid's head shot up. His brow was furrowed. "What are you talking about?"

"Er, about the body… uh." Shinichi stared fixedly down at his feet. "Long story short, I was investigating a drug deal, got caught by a crime syndicate, and was fed some mysterious poison that reversed my physical age down to, you know, six years old. I mean," he added in a rush as Kid's eyebrows climbed slowly up his forehead, "mentally, I'm eighteen, and I'm _actually_ eighteen too, so… if that makes you feel any better."

The way Kid was watching him had changed from sullenly depressed to tentatively intrigued. "You – are you joking?"

"No," Shinichi assured, maybe too quickly to sound entirely convincing, but he was sort of flying high from discovering the exact way Kid's face looked when he was surprised. The feeling of evoking a new emotion from Kid felt oddly similar to successfully solving a case. "My name is Kudou Shinichi."

"Mm." Kid smiled, just a faint little suggestion of amusement, but it was gratifying nonetheless. "Kudou Shinichi." Shinichi was ashamed to find that he was mesmerized by the way Kid seemed to taste each syllable, weighing the sounds on his tongue before he spoke them.

 _Getting a little too poetic there,_ Shinichi thought wryly to himself. He massaged his temples. This soulmate bond was serious business if it was making him this sappy.

The grounding sound of footsteps in the distance, accompanied by Nakamori's frantic shouting, made Kid straighten hastily and Shinichi glance at the door.

"You really should go," he told Kid, hurrying over to the door to find the hallway still empty, though the voices were drawing closer. "They're going to get here soon."

"I thought you said you were going to turn me in the police, Kudou Shinichi?" Kid taunted, and Shinichi sighed.

"Consider me letting you go this time a gift," he growled, and Kid laughed as he strode towards the door.

"Okay, my darling soulmate," he sang as he trotted out of the engine room, and Shinichi decided that yes, it really had to be a mistake, because Kid was _insufferable_.

He was considering trying to ignore the whole soulmate business and never see Kid again (according to most stories he'd heard, that was pretty much impossible once you'd met your soulmate in the flesh because you were constantly drawn to them and all, but Shinichi could probably manage it if he tried hard enough) when Kid stuck his head back through the doorway, grinning broadly. "I almost forgot to tell you. My name's Kuroba Kaito."

Shinichi, who had startled at his reappearance, lifted his eyebrows. "Are you sure you should tell me that?"

"Sure," Kid grinned, and _winked_. Shinichi hated that he found it charming rather than irritating. "You told me your secret identity, so I figured I should tell you mine, my darling soulmate."

"Stop _calling me that_ ," Shinichi insisted, but Kid was gone in a swirl of pink smoke and a flash of light before he could finish, and Shinichi had to sit down and groan into his hands.

* * *

When Kaito hesitantly typed "Kudou Shinichi" into the search engine, he definitely wasn't expecting… this.

Wide-eyed, he scrolled through the thousands ( _thousands_ ) of hits, mostly fanclubs or news articles praising Kudou Shinichi as the century's greatest investigative mind, even at the tender young age of eighteen. "The modern Sherlock Holmes" seemed to be a common theme. Most of the newer articles, however, were speculative of his current whereabouts – "The disappearance of Kudou Shinichi – where has the crime-solving genius gone now?" was one of the most popular hits.

Kaito sat back in his desk chair, blowing out a long breath. Well. He had _suspected_ , of course, that Shinichi was involved in detective work to some degree, but he hadn't actually thought Shinichi would be that well-known. He'd been wrong, apparently.

Sighing, Kaito absently clicked on one of the articles touting Shinichi as the "savior of the metropolitan police force." He scanned the wall of text listlessly, picking out words like "mental prowess" and "incredible deductions," before his gaze slid over to the accompanying picture and he did an involuntary double-take.

Because wow. Even in the grainy, low-quality photo, Kaito could see that Shinichi was gorgeous, all lean lines and dark hair and white-toothed smirks. He squinted at the computer screen, admiring the flash of Shinichi's eyes against his fair skin and the slope of his shoulders as he bent over a strip of crime scene tape, and sighed in a way that was definitely _not_ dreamy at all. Kaito didn't _do_ dreamy.

He went to sleep thinking about what it would be like to meet Shinichi, run his fingers through Shinichi's mess of hair and gauge the warmth of his skin with his own, and fervently wished his soulmate hadn't been a reckless idiot who got himself drugged and shrunken into a six-year-old body. Things would've been much easier if he hadn't.

* * *

"Good evening, my darling soulmate," Kid – or rather, Kuroba Kaito – called casually to Shinichi, extending a hand jovially towards him. Which in and of itself would be annoying and cause for bodily harm, but the fact that they were _free falling out of a helicopter_ makes it even worse.

"I _despise_ you," Shinichi hissed, swatting at Kaito even as he flailed, eyes drying, and wished desperately for a parachute to appear out of nowhere.

Kaito made an affronted noise – Shinichi rolled his eyes so hard he nearly broke something – and clutched at his heart. "I can't believe," he sobbed, "that my own _soulmate_ would _reject_ me like this –"

"Can you save the theatrics for a time other than now," Shinichi demanded, flapping in vain. The treetops were beginning to seem perilously close. He swallowed hard, throat chafing from lack of moisture. "Being rejected by me isn't going to matter when we both die in _three_ _seconds_."

"Now who's being theatrical," Kaito laughed before he barrel rolled gracefully through the air – Shinichi gaped for a full moment because _how_ (and also _wow_ but mostly _how_ ) – and swept Shinichi up in his arms. Shinichi opened his mouth to yell at him that it was _not the time for cuddling_ when Kaito opened his hang glider and they caught an updraft, veering sharply upwards.

The vertigo-inducing sight on the ground flying into the distance made Shinichi clutch at Kaito's gloved hands, his fingers tiny in comparison. "Whoa!"

"Isn't it a great sight?" Kaito beamed into Shinichi's ear, cutting a curvy line through the sky. "We should do this more often. Recreational hang gliding, I mean. Together."

"May I remind you that we _got thrown out of a helicopter by a murderer who was trying to frame you and this is not the time to be planning bonding activities_ ," Shinichi snarked, but he did pat Kaito's hand in agreement as he turned his face into Kaito's bicep. It really was beautiful – the feeling of Kaito's heartbeat synced perfectly to his and the wind in his face and the world stretching limitless below.

* * *

The first time Kaito met Ran (as himself, not Kid), he – well, he… he got insanely jealous.

( _Understandably_ _so_ , he would argue every time Shinichi brought it up later. How else was he supposed to have interpreted the situation? It had been a _natural reaction_.)

"So, Mouri-san, you're Conan-kun's… caretaker?" Kaito questioned, one eyebrow crawling up his forehead as he looked across the table at Ran. Behind her, he could see their waitress discussing something with a coworker and throwing hesitant looks in their direction.

Mouri Ran, beautiful specimen of grace and charm that she was, nearly _sparkled_ as she smiled at him and reached casually down to ruffle Shinichi's hair. "That's right. Conan-kun has been living with us since his parents went to the States for business." She beamed at Shinichi. "Isn't that right, Conan-kun?"

"Right!" Shinichi chirped, like some kind of overenthusiastic cartoon character, and practically blinded everyone within a ten kilometer radius with his gigantic smile.

Kaito stared. Maybe it was just him, but _he'd_ never elicited that sort of smile from Shinichi. The thought made him scowl into his cup of tea, storm clouds settling overhead.

Meanwhile, Ran was chattering away. "I heard you met Conan-kun at the park?" she asked brightly, resting her face in her hands as she beamed across the table at him. "It's so nice to see Conan-kun making new friends." Her expression, though, faltered for a split second, and Kaito instantly saw the tentative _but you're kind of weirdly older than him; are you some kind of predator_ that passed through her mind. He gritted his teeth.

"I did. He's a really smart kid," Kaito responded, smiling as naturally as he could manage (which was not all that naturally, to be honest). He swept his bangs out of his face. "But I'm sure you know that."

"Oh, of course I do. Conan-kun's a great little detective," Ran cooed, ruffling Shinichi's hair. Eyebrows lifted, Kaito waited for Shinichi to glare at her, but it never came. Instead, he just grinned up at her. Kaito's stomach churned unhappily at the sight.

"Anyway," Kaito began, angling his head at her, "how do you know Conan-kun, again?"

"He's the relative of a friend of mine," Ran answered, eyes taking on a distant glow. She looked like a painting of a Greek goddess or something. "I don't know where that friend went, though… Have you heard of Kudou Shinichi, by any chance? He's a little famous for solving murder cases."

Kaito, who had just taken a sip of his tea, choked, coughing violently instead. At Ran's side, Shinichi did the same, but masked it as a strained laugh when Ran cast a concerned look in his direction.

"Uh," Kaito started, slowly, trying hard not to look at Shinichi. "I… I think I've heard of him, yes."

"Really?" Ran sighed heavily, one hand migrating to press dramatically over her heart. "I don't know where he's gone, and I just – I miss him so much." She brushed at her eyes. "It's just really hard, you know? Waiting for him to come back."

Sneaking a glance over at Shinichi, Kaito found that he was watching Ran with an almost guilty expression, eyes regretful behind his glasses. In his lap, his small fingers curled into a pair of tight fists. He seemed – he seemed _devastated_.

Feeling as if someone had dumped a bucket of ice water over his head, Kaito stood with a clatter, almost overturning his mug. Both Shinichi and Ran's gazes snapped up towards him, Shinichi's bewildered and Ran's politely concerned.

"I'm not feeling well," he mumbled and kicked his chair back with the heel of his foot. Ran was now looking perplexed while Shinichi was starting to frown. "I think I'll leave."

"Kaito-niichan –" Shinichi began, but Kaito was already stalking out of the café. He was aware that he was acting like a child (ironically enough, considering which of them in their relationship was physically six), but he couldn't bring himself to care. It was infrequent, but not impossible, for someone to reject their soulmate in favor of someone else they already knew – it usually happened when people found their soulmates after getting married or while already in a serious relationship.

Thinking about it, Kaito didn't know why he'd thought Shinichi was unattached. With a face like that (his eighteen-year-old one, that was), there was no way he wasn't fending off suitors left and right. And Ran was _waiting_ for him to come back. Kaito didn't stand a chance.

Grumbling to himself as he tried to convince himself he could still find love after being rejected by his soulmate (it was at least possible in theory, if all the cheesy romance novels focusing on "love after soulmates" were any consolation), Kaito started down the sidewalk. He only made it a few storefronts away from Poirot when a tiny hand grabbed at his. "Hey, hold on. What the hell, Kuroba?"

Swallowing, Kaito turned to look down at Shinichi, who was glaring up at him with something close to irate exasperation. "What was that? Why did you just run out of there?"

"It's okay, Kudou." Tugging his hand out of Shinichi's grip, Kaito stared down at a crack in the sidewalk. "I understand if you don't want to…" He waved a hand, trying to find the words. "If you don't want to keep doing this."

When Kaito managed to look directly into Shinichi's face, Shinichi was gaping at him with a combination of mystification and horror. "What are you even talking about? Does this have something to do with Ran?"

With _Ran._ Hearing Shinichi calling her by her first name without honorifics made Kaito flinch. He took a step back, biting his bottom lip. "I mean," he mumbled, "it's pretty clear you and Mouri-san are… you know. With the – the first names and all that. And even if I'm your soulmate, if you'd rather… stay with her, or whatever, that's fine with me. I don't want to force you into anything, especially when she's waiting for you –"

"Oh my _God_ ," Shinichi interrupted, pinching the bridge of his nose with frustration. "Is _that_ what this is? You're jealous of _Ran_?" He laughed, disbelieving. " _Kaito_ , Ran's already found her soulmate. I mean, sure, for a while I had a crush on her –" (here, Kaito's heart did a complicated maneuver in his chest) "– but one, it wasn't anything serious because I knew she already had someone, and two, it was before I met _you_. Ran's just my childhood friend now – which, incidentally enough, is why I call her by her first name. And, you know, I just – I feel really bad about leaving her behind and living right next to her when she's always worrying about me." He rolled his eyes, though Kaito could see color rising in his rounded cheeks. "And if we're talking about someone waiting for me, aren't you doing the same thing?"

"Oh," Kaito said after a moment, feeling rather stupid. He opened his mouth, paused, and then shut it again. "Oh."

"O _h_ is right," Shinichi grumbled, and reached up to take Kaito's hand again. His hand was small enough that he could only curl his fingers around three of Kaito's. "Now, you're going to apologize to Ran and convince her that it's safe for me to be around you. She still thinks you might be some kind of child predator."

"Right," Kaito murmured. He glanced down at their entwined hands and noticed, belatedly, that if he placed his thumb over Shinichi's knuckles, their soulmate marks touched. He wondered if they had been placed for that reason.

* * *

"You shouldn't walk home by yourself, kid! It's dangerous!"

Rolling his eyes at the now familiar voice, Shinichi whirled around to find Kaito beaming at him from underneath a streetlight several meters away. "I _knew_ someone was following me. I didn't think it would be an idiot thief," he grunted, turning back around to continue down the sidewalk.

Spluttering, Kaito broke into a jog to catch up with him (which wasn't a particularly hard feat, considering how short Shinichi's legs currently were). "How _cold_ ," he sniped, shoving his hands into his pockets as he leveled a shit-eating grin down at Shinichi. "And here I was, worried about my darling soulmate."

"You don't _need_ to worry," Shinichi grumbled, definitely _not_ admiring the way Kaito's cheeks colored from the icy night air and his lips seemed obscenely red even in the murky light from the streetlamps. "I can get home from the professor's by myself. It's not that far of a walk."

Scoffing, Kaito reached down to tousle Shinichi's hair just because he knew Shinichi hated it. "But what would I do if you happened to get _kidnapped_ or something? Then where would I be?"

"Please, like anyone would want to kidnap me." Shinichi rolled his eyes and prepared to change topics. He wasn't expecting Kaito to halt abruptly, grab Shinichi's shoulders, and force Shinichi to meet his suddenly serious gaze.

"Seriously, though," Kaito said, his voice low and urgent and more than a little panicked, his eyes wide and earnest. "Don't joke about that. I don't know what I'd do if you actually got kidnapped. Just – I don't want anything to happen to you, okay?"

Taken aback, Shinichi could only nod, transfixed. He suddenly realized that the indigo ink of his soulmate mark matched Kaito's eyes exactly.

Exhaling, Kaito straightened. He pulled on his usual smirk so quickly Shinichi wondered if he had imagined the whole thing. "Great! Since you understand, I'm going to walk you home every night for safety reasons!"

"You _suck_ ," Shinichi grumbled, and nearly choked when Kaito sing-songed, "Not until you get your body back, my darling soulmate!"

* * *

Kaito was in the middle of making a sad birthday lunch of cup ramen and wishing someone (preferably Shinichi) would come visit (Hakuba and Aoko were at some Sherlock Holmes convention, much to Aoko's disappointment and many apologies), when the doorbell rang.

Blinking, Kaito got up from the kitchen table to go open the door. For a moment, he didn't see anyone until he looked down to find Shinichi glaring up at him from the doormat with a giant white box under his arm.

"You didn't tell me," Shinichi hissed, "that it was your _birthday_." He glared. "I felt like I'd forgotten something this morning, probably because of the whole soulmate thing, and I had to _call Nakamori-san to confirm_."

"Er… uh," Kaito stuttered as Shinichi huffed and shouldered past him, toeing off his shoes on the genkan as he muttered under his breath about the incompetence of stupid soulmates. "I – didn't want you to – I thought you'd be busy?"

"It's your _birthday_ ," Shinichi insisted, rolling his eyes. He stalked into the kitchen, side-eyeing Kaito's cup ramen. "You're supposed to celebrate."

"Says the guy who always forgets his own birthday. If Ran-san hadn't told me it was your birthday last month, you wouldn't have done anything," Kaito reminded him, leaning against the doorway to watch as Shinichi set the box he'd been holding on the counter.

Shinichi waved a hand. "That's different. When it's the birthday of someone important, you _have_ to celebrate." Before Kaito could do more than scowl and open his mouth to demand if Shinichi was implying he wasn't important, because that was the biggest lie of the century, Shinichi opened the box to reveal a perfect two-tiered chocolate cake with chocolate frosting and _Fireworks!_ written across the top in blue icing. Kaito's breath caught.

"I had to get this at that bakery next to the police station," Shinichi was saying as Kaito stumbled over. "You wouldn't _believe_ the weird looks I got while trying to buy this. Apparently it's not normal for a seven-year-old to waltz into a bakery and demand their biggest chocolate cake. Also, they thought writing 'Fireworks!' on the cake would be weird and started asking _questions_ –"

Kaito cut him off by through his arms around Shinichi's shoulders. It was a bit of a stretch, but worth it to feel Shinichi startle but still lean back into him.

"Thank you, Shinichi," he murmured into the watermelon-shampoo-scented hair above the arm of Shinichi's glasses.

He felt Shinichi's smile rather than saw it. Shinichi's hand, the one with _Yo, kid. What are you doing in a place like this?_ scrawled across the knuckles, lifted to curl around Kaito's forearm. "Happy birthday, Kaito."

* * *

"What if something goes wrong?" Kaito fretted as Haibara did a last check-over of Shinichi's body. He was leaning against Agasa's kitchen counter, looking a lot more nervous than Shinichi himself felt and twisting his fingers together into what looked like a Gordian knot.

Haibara sighed, placing her hands on her hips. She was terrifying as a six-year-old, Shinichi thought as he pulled his glasses off for (hopefully) the last time. Haibara as Miyano Shiho would be beyond horrifying. "Look, Kuroba-kun," she growled, "You need to calm down and have a little faith. I understand that Kudou-kun is your soulmate –"

"Literally the other half of my soul," Kaito interjected, eyes boring into Shinichi's skull. " _Literally_. The other half of my soul, whom I haven't gotten to touch properly for _a year_ –"

"And if you ever want to, you'll let me take the antidote," Shinichi jumped in, lifting his eyebrows.

Kaito looked chagrined. "Even if it doesn't work, you know I'll still stay with you," he told Shinichi solemnly, and Shinichi smiled dopily back at him, because the universe really had blessed him with the best soulmate –

"Oookay, if you two are done being sickening, we should get started with the antidote," Haibara snapped, and Shinichi rolled his eyes even as he reached out to take the familiar red-and-white pill from her. Just because _she_ hadn't met her soulmate yet…

Settling back against the counter, Kaito continued to look uneasy. "You promise it's safe," he asked Haibara, and Haibara groaned, long-suffering.

"Like we told you, the antidote is going to hurt, because it's literally _reshaping his bones_ , but yes, it's _safe_ ," she informed him for what felt like the eighteenth time. Shinichi rolled his eyes when Kaito scowled at her.

"Okay, I'm taking it," he said loudly, and swallowed the pill.

It really did hurt, Shinichi thought in one corner of his brain as he collapsed onto the floor. It felt as if someone had doused his insides in gasoline and set them on fire; something was clawing its way through his stomach, his chest, his throat –

He was distantly aware of Kaito gathering tenderly him in his arms and yelling something like, "You said it was safe!" at Haibara and Haibara responding indignantly with, "I told you _it would hurt_ , you imbecile," before he blacked out entirely.

When Shinichi came to, it was in his real body and Kaito was clutching him to his chest and beaming down at him as if he was the most precious thing in the world. The pad of Kaito's thumb, with its familiar _Fireworks!_ written in Shinichi's own handwriting, swept across Shinichi's sweaty temple in a soothing pattern. Smiling weakly, Shinichi reached up to grasp Kaito's hand and watch, entranced, as their marks lined up perfectly.

"Welcome back, my darling soulmate," Kaito whispered, and Shinichi smirked up him and said, "Fireworks!" just to see Kaito break into a grin.

* * *

 **Sap is my specialty. Yeah.**

 **Hope you enjoyed this monstrosity even a little (if you did, please consider leaving me a review!) and I'll see you all with (hopefully) another chapter of _Keep Me Warm_! - Luna**


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